Python 2.3.7 data type double

Python 2.3.7 has the following functions as number converters: float(num), int(num), long(num). But what happened to double(num)?
If my number is a=3.33333 and do round(_,2), the output I get is 3.3300000000000001. I was expecting to see 3.33.
What’s happening to here?
Thank you.
liliya

I'd like to have decimal module on 2.3.7 version, because the project I will work on in near future is developed in this version, other versions have compatibility problems.
So there is no decimal module for 2.3 anywhere?